Chemistry / Unit 2: Atomic Structure
2.3 Atomic Number, Mass Number and Ions
Notes
Two numbers describe each atom.
- Atomic number ($Z$) = number of protons.
- Mass number ($A$) = protons + neutrons. So neutrons $= A - Z$.
- Notation: $^{A}_{Z}\text{X}$, for example $^{23}_{11}\text{Na}$ has 11 protons and 12 neutrons. You can also write sodium-23.
- An ion has gained or lost electrons. Cations are positive (lost electrons); anions are negative (gained electrons).
- Example: $\text{O}^{2-}$ has 8 protons and $8 + 2 = 10$ electrons.
Practice
How many neutrons are in an atom of $^{56}_{26}\text{Fe}$?
Neutrons $= 56 - 26 = 30$.
How many electrons are in a $\text{Ca}^{2+}$ ion? (Calcium's atomic number is 20.)
A 2+ charge means 2 electrons were lost: $20 - 2 = 18$.
An atom has 17 protons, 18 neutrons and 18 electrons. What is it?
17 protons means chlorine. One extra electron gives a 1- charge, and $A = 17 + 18 = 35$.
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